Midterm #2 (Spring 1997)
†(15 points) Define and explain the following terms:
two army problem
(15 points) Error correction and detection.†
What is the minimum number of error recovery bits required to
recover from an n bit error?† Explain
why this number of bits is sufficient.
If the ratio of error detection bits to payload bits remains
constant, what are the tradeoffs between having the error bits applied to a
single character vs. a block of characters?
†(20 points) One way to
support mobile hosts is by assigning a host a temporary IP address in its new
location, and continuing to use its
old address by tunneling IP through IP back to an agent running on the mobile
hostís home network.
Explain the steps required to permit a mobile host to register
on a local network and start receiving packets using its permanent IP address.
Give one advantage and one disadvantage of this approach to IP
mobility compared with having a mobile host only use its permanent IP address.
When tunneling IP through IP, it is not possible to always
maintain a one for one match between tunneled packets and the native packet.
Why?† Explain the potential performance
implications of this limitation.
†(15 Points) IP
The Internet is running out of IP addresses to assign, but
there are substantially less than four billion hosts on the Internet.† Explain why this is happening.
Joe Hacker reads about IPv6†
(with 128 bit address) and is concerned about routing table size.† He decides that the best solution is to
encode latitude and longitude into IP addresses. Explain how this might be used
to reduce routing table size.† Provide a
reason why this might not reduce routing table size.
†(15 Points) Consider a
network where on average, messages between two hosts must travel through 10
routers (and thus 11 hops).† On average,
one packet in every 1,000 is lost as is passes through a router.
If one packet in 100 is garbled per hop, what is the
probability of a packet making it through the network if no link level
re-transmission is used?† What if
link-level re-transmission is used?
Repeat part a if the probability of a link garbling a packet
is 1 in a billion.
†(15 points)† Congestion Control
Why is congestion control more important in a network carrying
traffic whose bandwidth requirements vary dynamically during a session compared
with fixed bandwidth traffic?
In TCP (Jacobson) congestion control, the variance in
round-trip times for packets implicitly influences the congestion window.† Explain how a high variation in the
round-trip time affects the congestion window.†
What is the impact of this high variation on the throughput for a single